In recent years, the demand for high-capacity secondary batteries, especially nonaqueous secondary batteries making use of lithium intercalation and deintercalation reactions, has been increasing as a power source of electronic equipment, such as potable telephones, video cameras and notebook personal computers. With the advancement of the functions of such electronic equipment, the secondary batteries have been desired to have higher capacity. The inventors of the present invention have previously achieved a high capacity by using a specific metal oxide having high lithium ion intercalating power as a negative electrode active material. However, since the ability of the positive electrode material remains non-improved, the proportion of the positive electrode active material to be charged in a battery must be increased in order to enjoy the high capacity, which means that the positive electrode sheet is to have an increased thickness.
A nonaqueous secondary battery usually comprises a positive electrode sheet and a negative electrode sheet which are laid one on another with a separator therebetween, rolled up, and put in a cylindrical battery case. In order to roll up the sheets into a true circular cylinder without generating cracks, the positive and negative electrode sheets must be flexible. In order to obtain a high capacity, the active material content in the electrode material mixture should be increased as high as possible. It follows that the electrode sheet has insufficient flexibility, and the thicker the layer of the mixture, the more liable is the sheet to generate cracks and the more liable is the active material to fall off the sheet. Therefore, it has been required to use a positive electrode sheet having improved flexibility in a system using a metal oxide negative electrode active material as in the present invention.
It has been proposed to coat a metallic current collector with an electrically conductive coating film comprising an electrically conductive filler, such as carbon black or metal powder, and a binder to prevent fall-off of the active material as disclosed in JP-A-63-121265 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but the flexibility obtained by the electrically conductive coating film was still insufficient.